The present invention relates to communication systems and methods, and, more particularly, to a communication system comprising a master/slave structure, to a master unit for such a communication system and to a method for operating a communication system comprising a master/slave structure.
In manufacturing and automation technology, serial bus systems are increasingly used in which peripherally arranged devices of a machine periphery such as I/O modules, transducers, drives, valves and user terminals communicate via an efficient real time communication system with automation, engineering and visualization systems. In this process, all subscribers are networked via a serial bus, e.g. a field bus, the data exchange via the bus being performed on the basis of the master/slave principle.
The active subscribers in the bus system, typically controlling devices, possess a bus access authorization and determine the data transfer on the bus. In the following, the active bus subscribers will be designated as the master units in the serial bus system. Passive bus subscribers, on the contrary, are generally peripheral machine devices. They do not have a bus access authorization, i.e. they can only accept received information signals or transmit information signals to a master unit upon request from the master unit. In the following, the passive bus subscribers will be named the slave units in the serial bus system.
Field bus systems comprising a master/slave structure are generally arranged in a ring topology to avoid complex wiring, whereby all bus subscribers are connected to a ring-shaped transmission path. A data signal generated by the master unit is fed into the ring-shaped transmission path by the master unit and consecutively cycles through the slave units serially connected to the ring-shaped transmission path to be again received and evaluated by the master unit.
The data signals are usually organized as data packets by the master unit, the data packets being composed of control data and user data, whereby the Ethernet standard is employed allowing for data packets with a length of up to 1500 byte at a simultaneously high transmission rate of 100 Mbit/sec. Each of the slave units connected to the ring-shaped transmission path then processes the user data of the Ethernet telegrams intended for it, the user data being fed into the ring-shaped transmission path by the master unit.
The master/slave communication systems comprising a ring structure are typically configured in such a way that the master unit comprises a transmitting unit as a data launching point and a receiving unit as a data lifting point. The individual slave units are then connected at the transmission path to form a ring, whereby each subscriber is connected to two neighbours and the first and the last subscriber in the ring is connected to the master unit. The transmission of the data packets is thereby carried out in one direction starting from the master unit via its transmitting unit to the first connected slave unit and from there to the next one up to the last slave unit in the ring in the direction of the data, and then back to the receiving unit of the master unit.
To receive the cycling data packets from the previous subscriber, each slave unit comprises a first terminal and a second terminal for forwarding to the subsequent subscriber, a processing unit being arranged between the two terminals to process the data packets cycling through the slave unit.
A critical requirement set for the master/slave communication system, particularly for use in manufacturing and process automation, is a high failure tolerance, i.e. the ability of the communication system to perform the desired function, e.g. manufacturing a work piece, in spite of the occurrence of a failure. Failures in the communication system that have to be dealt with out affecting the manufacturing process comprise, apart from failures in the data packets, particularly the failure of a subscriber in the transmission path and/or an interruption of the transmission path, e.g. by physically cutting through the transmission medium.
To achieve a failure-tolerant master/slave communication system, particularly in the case of link failures, i.e. if entire transmission sections fail, double-ring structures are frequently used. U.S. Pat. No. 4,663,748 thus describes a communication system comprising a master/slave structure in which the master unit is serially connected to a plurality of slave units via two communication paths operating in contra-sense. The master unit thereby comprises two transmitting and receiving units comprising respective transmitters and/or receivers and associated control units to allow for data packets to simultaneously cycle through the two communication paths. The slave unit then comprises a coupling unit to reconfigure the communication system if a link failure occurs, e.g. in the case of a broken communication line, by monitoring the data packets on the two communication paths and by respective switching, reconfiguration taking place in such a way that a failure of a larger section of the communication system caused by the link failure, or even a complete failure, may be prevented.
Failure-tolerant master/slave communication systems having a double-ring structure in which the master unit comprises two respective transmitting and receiving units having corresponding transmitters and/or receivers and associated control units to launch data packets on both communication paths, provide a higher hardware and switching complexity of the master unit and thus considerably increase costs.